1. Field of the Invention
The invention relates to a process for the photoelectrochemical etching of semiconductors and to the specific metal-ion treatment of the semiconductors, which can also be used in photoelectrochemical cells or as photovoltaic cells in solid-state applications.
2. Description of Prior Art
Semiconductors are commonly etched for purposes of improving the surface of the semiconductors. Although etching may serve many purposes, there are two primary areas in which etching is useful. First, etching serves to remove "damaged" surface layers from semiconductors. Second, in "geometrically selective etching", a specific geometric pattern is produced on the semiconductor.
The removal of damaged surface layers by etching is useful in preparing photovoltaic devices in which damaged or less perfect (less than the bulk) surfaces may give rise to undesirable surface states resulting in lowered output parameters, particularly photovoltage and fill factor, and having fewer surface recombination centers, which results primarily in reduced photocurrent. For such uses, the etchant is normally a reagent which attacks the semiconductor chemically, such as by dissolving it or by oxidizing it to form products which are subsequently dissolved at a controlled rate. In this context, particular attention is directed to the exhaustive etching of GaAs crystals disclosed by B. A. Parkinson, A. Heller and B. Miller, Applied Physics Letters 33(6) 521 (1976) by nonconvective etching with H.sub.2 O.sub.2 /H.sub.2 SO.sub.4. By using this technique an initially shiny crystal face is converted to a matte non-reflecting surface. As was noted above, this is particularly desirable for photovoltaic applications where matte surfaces reduce light reflection and hence result in higher photocurrents as a result of increased light absorption. Additionally, the treatment of the etched GaAs with ruthenium ions has been shown to increase both the photovoltage and fill factor of a photoelectrochemical cell utilizing the GaAs as a photoanode in a polyselenide electrolyte.
Geometrically selective etching is often radiation induced and, although related to the method of the invention, is performed for a different purpose such as recording holographic data or forming diffraction patterns. Thus, in 1956, A. Uhlir [Bell System Tech. J. Vol. 35,33 (1956)] first reported electrolyte shaping of Ga and Si by localized illumination to inject carriers in a geometrically controlled fashion and was able to produce a pattern of concentric dimples. In 1966, Yu. V. Pleskov (U.S.S.R. Patent 190,758) selectively cut n-GaAs crystals by photoelectrochemically etching them to obtain a desired surface profile.
Hologram recordings on a polished Si surface were reported by A. L. Delisa et al [Appl. Phys. Letts. 17, 208 (1970)], and were performed by photoanodically etching the Si surface in a 5% aqueous HF solution. Diffraction patterns on n- and p- crystals and on AlGaAs epitaxial layers, were formed by forming a two beam interference pattern from an HeNe laser onto a semiconductor surface which was etched at the same time with an aqueous solution of an oxidizing agent [L. V. Belyakov et al, Soviet Phys. Tech. Phys. 19, 837 (1974)]. Hologram recordings were effected in CdSe sputtered films on glass by a photoetch process wherein the CdSe film was illuminated by a laser in aqueous HNO.sub.3 or K.sub.3 Fe(CN).sub.6 solution [Sterligov, V. A. and Tygai, V. A., Soviet Tech. Phys. Letts. 1, (1975)].
Finally, P. D. Greene studied the photoelectrochemical etching of GaAs by Fe.sup.+3 based etches and found that n-GaAs was etched preferentially compared with p-GaAs and that the rate of etching of n-GaAs depended on the doping concentration [Inst. Phys. Conf. Ser. No. 33A, 141 (1977)].